This invention relates in general to valves adapted for pressure relief service and having provision to surpress vibrations. More specifically, it encompasses relief valves operable with incompressible fluids, the valves achieving full lift with small overpressure accumulation and closing at a uniform adjustable pressure below the set point. These features are achieved without restricting the maximum flow capacity of the valves.
In pressurized fluid systems such as the pressure vessels commonly used in the petrochemical industry, it is necessary to provide a safety relief valve that will relieve pressure from the system during an overpressure condition by venting fluid from the system. Optimally, the valve should open when the system pressure exceeds a predetermined set point or pressure and quickly reach a full open or "full lift" position without an excessive overpressure build up. Also, the valve should close as soon as it has vented the minimal volume of fluid that is sufficient to return the system pressure to a predetermined safe level below the set pressure. A rapid closing or "blow down" prevents an excessive loss of fluid from the system. In practice, these objects are more difficult to achieve with incompressible fluids than with compressibly fluids.
Conventional safety relief valves employ a disc that engages a valve seat formed on a nozzle. A spring is adjustable to counterbalance the system relief set pressure. When the system pressure exceeds the set pressure, the disc, and typically a disc holder that together form a valve head, move upwardly against the force of the spring. It is common to employ a disc holder and disc that overhang the nozzle. Thus when the system pressure is sufficient to crack the valve open, it acts on the additional overhang area to provide an additional force for lifting the valve. While this overhang area can be useful in causing a quick opening of the valve, the fluid forces on it tend to resist closing until the system pressure has decayed to a value substantially below the set pressure.
While valves of this generaly type, described for example in U.S. Pat. Nos. 2,880,751 and 3,854,494, perform satisfactorily with compressible fluids such as steam and air, they are generally unstable and vibrate excessively when used with incompressible fluids such as water, oil and liquids generally. The conventional relief valve for liquid service does not achieve full lift and therefore its maximum rated flow capacity until the overpressure reaches a level in the range of 15% to 25% above the set pressure. Also, the closing pressure is typically not controllable and is well below the set pressure of the valve.
Moreover, when many conventional relief valves are adjusted to operate close to the set pressure, they become dynamically unstable and vibrate in a violent and destructive manner. The vibrations are typically characterized as "chatter", "flutter", and "hammer." Chatter occurs when the valve opens but remains in close proximity to the nozzle seat. Flow induced vibrations then cause the valve to strike the seat repeatedly. This condition can quickly destroy the integrity of the valve seat. Fluttering describes valve cycling due to flow induced vibrations where there is no metal-to-metal contact. Hammering occurs when there are broad fluctuations in the system pressure causing the valve disc to lift substantially and then to slam against the valve seat. These conditions are generally indicative of an unstable valve that will require frequent maintenance such as replacement of main seats, guiding surfaces and bellows. Also, such conditions can eventually destroy the valve, work it loose at its fittings and damage the associated equipment.
Another problem with relief valves is that when they are used with gases carrying moisture, the rapid expansion of the gas as it leaves the nozzle can cause the moisture to form as ice on the valve seat. This ice may prevent the valve from closing properly.
Hitherto, solutions to the stability problem have centered on mechanisms to retard movement of the valve head or to choke the fluid flow. U.S. Pat. No. 2,792,015 to Smith is an example of a liquid relief valve that employs a flow restriction in the nozzle. When valves with such restrictions are used in a system it is frequently necessary to employ a larger valve than normal to compensate for the restrictions and thereby to achieve the desired flow capacity.
The Smith valve also utilizes a valve member that overhangs and projects downwardly from the nozzle to generate a "pop" to full lift and to guide the flow. U.S. Pat. No. 3,520,326 to Bowen et al and U.S. Pat. No. 3,572,372 to Moore describe spring loaded relief valves that include adjustable nozzle rings that guide the flow in cooperation with surrounding, downwardly projecting conical surfaces formed on members secured to the disc or a disc holder. In these structures the downwardly projecting valve surfaces either have an insignificant effect on the flow rate as compared with the orifice at the main seat, or they affect the flow rate as a substantially linear function of the valve position. The fluid forces acting on these downwardly projecting surfaces vary as a linear function of the distance the valve disc has lifted off the seat.
Canadian Pat. No. 797,570 describes a skirt member secured on a disc holder that together with a nozzle ring defines a "huddle chamber" to control fluid pressures in the valve. In this valve, however, the flow is increasingly restricted as the valve moves toward its fully open position.
It is an object of this invention to provide a disc-type valve structure useful in a spring-loaded relief valve for liquids as well as gases, that automatically and quickly achieves full lift or pop-type opening to a full lift position with a relatively small overpressure, reseats at an adjustable pressure just below the set pressure, and is dynamically stable.
Another object is to eliminate the problem of icing at the seat in such a valve.
A further object of this invention is to provide a relief valve that does not choke the flow or otherwise restrict the full flow capacity, thus having a desirable "gain characteristic" defined as the relationship of flow rate to valve travel.
Another object is to provide a liquid relief valve that achieves full lift within 10% of set pressure and does not chatter, flutter or hammer.
Still other objects are to provide a relief valve having the advantages hereinafter appearing, that requires relatively low maintenance and repairs and has a long product life and a cost of manufacture comparable to that of conventional relief valves.